If you want to hear my take on it, consider this. The lighter bullet results in a lead savings of 1,000 grains per box, five boxes saves almost a pound of lead, a case of 20 is going to save about 4, and I'm pretty sure that their costs for lead will be maybe fifty cents per pound, since salvage price runs about 60 cents even in small batches. When small batches of lead cost 60 to buy, then need refining and alloying, even bought in many tons at a time, can it really be sold for much less than that? I just don't know.
Even if it is bought for only 30 cents a pound, that's over a dollar in savings, going from a 130+- load from the 158 or even 148.
Will a company that sells millions, literally millions of rounds of ammo every month or so, willingly sell them with more materials than necessary? Seriously, not a chance. The big companies have always, for many decades, tested every new powder that is created, just to see if they can shave off half a grain in the charge without significantly impacting performance.
Want to know why the plated bullet is so popular and is being promoted? A fraction of a cent worth of copper, a tiny bit of energy and work, and essentially another fraction of a cent saved results in a lead bullet selling for a penny more if it's plated. compare that to jacketed? copper probably costs three to four times as much as lead, and seriously, buying sheets of copper and drawing jackets will be extraordinarily more expensive than a simple plated bullet.
Follow the money. These miniscule alterations in operations and materials cost save millions of dollars annually. Constant design alteration that create products that are perceived as being far better than ever allow for premium rounds to go at a much higher price, cutting costs by even a fraction of a cent adds up to a fortune in the long run.
Have you noticed that a bottle of water now comes in a floppy, thin container, and has a lid as big as a beer cap? boy, I'd love to hav even a tiny fraction of the money they save annually just because they reduced the price of their manufacturing.